1. Field of the Invention
Embodiments of the present invention relate to a load cell for a downhole load measuring tool, for example to measure the tension or compression applied to an oil well logging string by a wireline or tractor tool.
2. Description of the Prior Art
Well logging engineers often require a load measuring sensor to be fitted between the top of a logging toolstring and the bottom of a wireline cable, for example on the cable head. This monitors the axial load, which is being applied directly to the toolstring. This is an important measurement to determine whether the toolstring has become stuck down the well and/or if the tension in the wireline is approaching the breaking point of the cable head. The cable head load sensor also helps the logging engineer to determine the forces being exerted on the toolstring by the wireline and the tractor during operations in highly deviated wells.
A challenge faced when designing a cable head load unit is to decouple the axial load created by downhole hydrostatic pressure alone, from the load applied by the wireline or tractor. In a deep well the hydrostatic pressure will result in tens of thousands of pounds of axial compressive force to be applied to the toolstring and this is an order of magnitude greater than the maximum potential wireline force.
US 2008/0216554 discloses a downhole load cell which acquires downhole pressure and temperature information. This downhole pressure and temperature information is used to calibrate the downhole load cell readings. However, it would be desirable not to have to take temperature and pressure measurements and then have to temperature and pressure calibrate the load cell as this increases manufacturing time and requires the load cell to have compensation files which need to be prepared and used when measurements are made.